1. Field of the Invention
This invention relates to catalyst compositions. In particular, it relates to shape-selective photoassisted heterogenous catalyst compositions which comprise a zeolite having a photoassisting species such as a photocatalytic, photoactive, or photosensitive material, within the zeolite structure.
2. Background Art
Large-scale industrial reactions are generally promoted by means of a catalyst or by the addition of energy in the form of heat to the reaction. One form of energy that has recently come into commercial chemical synthesis is radiant or light energy. Addition of radiant energy to a reaction can produce electronically excited molecules which are labile and therefore capable of undergoing chemical reactions.
Organic molecules which have all their electrons in stable orbitals are in the ground electronic state. Such orbitals may be bonding or non-bonding. If a photon of sufficient energy collides with the molecule, and is absorbed, one of the electrons is promoted to an unoccupied antibonding molecular orbital at a higher energy level.
Such excitation of an electron causes a redistribution of valence electrons resulting in internuclear configuration changes. Consequently, an electronically excited molecule can undergo chemical reactions that are quite different from a corresponding ground state molecule.
A photon's energy is defined in terms of its frequency or wavelength, EQU E=h.nu.=hc/.lambda.
where E is energy; h is Planck's constant, 6.6.times.10.sup.-27 erg-sec; .nu. is the frequency of the radiation, sec.sup.-1 ; C is the speed of light; and .lambda. is the wavelength of the radiation. Upon absorption, the absorbing species receives all of the energy of the absorbed proton. The effects of absorption on the absorbing species usually depend on the wavelength of the incident radiation. Radiation chemistry involves energetic photons, usually of wavelength less than 1000 A which result in ionization of the absorbing species. Photochemistry relates to photons whose energies lies in the ultraviolet region (1000-4000 A) and in the visible region (4000-7000 A) of the electromagnetic spectrum. The absorption of such a proton causes a molecule to become electronically excited. Radiation resulting from longer-wavelength photons is known as heat. Such radiation primarily excites vibrational modes in a molecule.
The present invention relates to chemical reactions involving photons of ultraviolet or visible wavelength. Where such absorbed energy is released as chemical energy, fragmentation, free-radical formation, isomerization, and addition reactions may take place in the photochemical system. Unlike thermal reactions which occur between any molecules whose total energies are above some minimum level, the degree of excitation of reactants may be precisely controlled in photochemical reactions. Since the molecules are imbued with additional electronic energy upon absorption of light, reactions can occur along completely different potential-energy surfaces from those encountered in thermally excited systems. Greater amounts of light energy may be absorbed by a molecule than is possible thermally, since thermal excitation often involves other competing processes, such as bond rupture, which occur before the desired energy state can be reached. Consequently, unique, thermodynamically unstable, structurally strained molecules may often be formed photochemically, but not thermally. Similarly, photolytic decomposition can occur at low temperatures, resulting in large concentrations of reactive intermediates which may be trapped and studied with conventional techniques.
A general characteristic of radiant energy absorption is the capability of acting directly upon specific molecules in the system, rather than adding energy to the system as a whole. Such specificity allows the system as a whole to remain at a relatively low temperature, since only the reactants need be activated. This furthermore permits the use of solvents which could not be used in a thermal reaction.
Although the foregoing photochemical reaction systems are capable of selectively reacting specific components in the reaction medium, it is difficult to insure that the electronically excited reactants form the exact products which are desired. For example, hydrocarbons which are electronically excited by ultraviolet radiation may form both branched and straight-chain products. This production of unwanted by-products reduces the overall yield of the desired product. Furthermore, if only straight-chain molecules are desired, they must ordinarily be separated from the product mixture, a time-consuming and costly process. A shape-selective catalyst composition which contains a photoassisting species, i.e., a species which enhances catalytic activity upon exposure to ultraviolet or visible radiation, by its very nature, will selectively photocatalyze only those molecules within a mixture of potential reactants, having the "proper" shape. Thus, such a photoassisted heterogenous catalyst composition is selective for reactants as well as reaction products. Consequently, it is apparent that a photoassisted heterogenous catalyst composition which is also shape-selective would be of great value in the reaction of and production of certain molecules.